1. Field of the Invention
The present invention relates to a liquid crystal display apparatus, a driver for a liquid crystal display panel, and a liquid crystal display panel driving method. More particularly, the present invention relates to a technique for suppressing quality deterioration of a display image which is caused by an offset voltage of an amplifier integrated in a driver of the liquid crystal display panel.
2. Description of the Related Art
One of technologies, which are most widely used to drive a liquid crystal display panel, is an inversion drive method. The inversion drive method is one involving inverting the polarity of a data signal supplied to a data line (signal line) at predetermined spatial cycles and time cycles in order to prevent the so-called burn-in phenomenon. Note that, in this specification, the polarity of a data signal is defined relative to a voltage level (common voltage) of a common electrode of the liquid crystal display panel. When a data signal has a signal level higher than a common voltage VCOM, the polarity of the data signal is defined as a “positive” polarity. In contrast to this, when a data signal has a signal level lower than the common voltage VCOM, the polarity of the data signal is defined as a “negative” polarity. The inversion drive method involves reducing a direct current component of a voltage applied to a liquid crystal capacitor of a pixel to effectively prevent the occurrence of the burn-in phenomenon.
In the inversion drive, various cycles for inverting the polarity of the data signal may be selected. In a dot inversion drive method which is one of the most typical examples of the inversion drive method, data signals whose polarities are opposite to each other are written into adjacent pixels in any one of the vertical direction and the horizontal direction. That is, in the dot inversion drive method, the polarity of the data signal is inverted for each pixel in any one of the vertical direction and the horizontal direction. When a large-size liquid crystal display panel is driven, in many cases, although the polarity of the data signal in the horizontal direction is inverted for each pixel, the polarity of the data signal in the vertical direction is inverted every two pixels. In this specification, the inversion drive method in which a cycle, at which the polarity of the data signal in the vertical direction is inverted, corresponds to α-pixel(s) is referred to as an αH inversion drive method. For example, the inversion drive method of inverting the polarity of the data signal in the vertical direction for each pixel (as in the dot inversion drive method) is described as a 1H inversion drive method. In addition, the inversion drive method of inverting the polarity of the data signal in the vertical direction every two pixels (as in the dot inversion drive method) is described as a 2H inversion drive method.
The data signal is normally generated as follows. A driver for generating a data signal (which is often called a source driver) includes a grayscale voltage generating circuit, a D/A converter, and an output amplifier, which are integrated therein. The grayscale voltage generating circuit generates a set of grayscale voltages having voltage levels respectively corresponding to grayscale levels which a pixel can express. The D/A converter selects a target grayscale voltage from the set of grayscale voltages based on display data and outputs the selected grayscale voltage to the output amplifier. The display data is data indicating a grayscale level of a pixel to be driven. The output amplifier outputs, to a data line, a data signal having a voltage level equal to the grayscale voltage supplied from the D/A converter. A differential amplifier in which an output terminal of an output stage thereof is connected with one of two input terminals of an input differential stage thereof, that is, a voltage follower is used as the output amplifier in many cases.
In general, in order to generate the grayscale voltage in the grayscale voltage generating circuit, a resistance ladder and an amplifier (operational amplifier) for supplying a bias voltage to the resistance ladder are used. A set of grayscale voltages are generated by dividing the bias voltage by the resistance ladder. The bias voltage outputted from the amplifier connected with the resistance ladder is determined such that the grayscale voltage becomes a voltage level reflecting a γ-curve of the liquid crystal display panel. Therefore, the amplifier connected with the resistance ladder is often called a γ-amplifier. In many cases, a voltage follower is used as the γ-amplifier.
A problem regarding the driver of the liquid crystal display panel is that the amplifier integrated therein has an offset voltage and thus a voltage actually outputted from the amplifier may be different from a target value. For example, when the output amplifier has the offset voltage, the voltage level of the data signal is deviated from the target value, so a voltage written into the pixel is deviated from the target value. This causes an actual grayscale level of the pixel to be different from a target grayscale level to deteriorate the quality of an image. In particular, when the offset voltage varies for each amplifier, an offset problem is serious. This is because a variation in offset voltage is recognized by the human eye as longitudinal streak-like unevenness extending in a data line direction. Similarly, when the γ-amplifier has the offset voltage, the actual grayscale level of the pixel is deviated from the target grayscale level to deteriorate the quality of the image.
An effective method of avoiding the problem of the offset voltage of the amplifier is that the polarity of the off set voltage is inverted at appropriate cycles. Note that, in this specification, the polarity of the offset voltage means a magnitude relationship between a voltage desired to be outputted from the amplifier (hereinafter, referred to as “target voltage”) and a voltage actually outputted from the amplifier (hereinafter, referred to as “actual voltage”) and thus is different in concept from the polarity of the data signal. When the polarity of the offset voltage is inverted at appropriate cycles, it is possible to prevent the influence of the offset voltage from being sensed by the visual sense of human. Hereinafter, when the actual voltage is higher than the target voltage, the polarity of the offset voltage may be referred to as the “positive polarity”. In addition, when the actual voltage is lower than the target voltage, the polarity of the offset voltage may be referred to as the “negative polarity”.
As compared with a reduction in offset voltage, it is technically easy to invert the polarity of the offset voltage. This is a more practical approach. The offset voltage of the amplifier is caused mainly by a variation between threshold voltages of a pair of MOS transistors included in the input differential stage and a variation between threshold voltages of a pair of MOS transistors included in an active load (for example, a current mirror circuit) connected with the input differential stage. Therefore, for example, when a connection relationship between the input terminal of the amplifier and the pair of MOS transistors included in the input differential stage and a connection relationship between the pair of MOS transistors included in the active load are changed, the polarity of the offset voltage can be inverted while the offset voltage is maintained at the same amplitude.
To be more specific, a technique for alternately using a pair of MOS transistors of an offset input differential stage at a cycle corresponding to four frame periods to invert the polarity of the offset voltage, thereby avoiding the problem of the offset voltage is disclosed in JP 11-305735 A (see, for example, paragraph [0125]).
A technique for inverting the polarity of the offset voltage every predetermined number of lines during a predetermined number of frame periods, thereby avoiding the problem of the offset voltage is disclosed in JP 2002-108303 A. JP 2002-108303 A discloses, for example, that, when a frame period includes eight lines, the polarity of the offset voltage is inverted every seven horizontal lines to cancel the offset voltage at a cycle corresponding to 14 frame periods.
In order to further improve the image quality, as disclosed in JP 11-249623 A, it is suitable to invert the polarity of the offset voltage every predetermined number of horizontal lines during each frame period. JP 11-249623 A discloses a technique for inverting the polarity of the offset voltage every n-horizontal lines during each frame period and every n-frame periods, thereby avoiding the problem of the offset voltage. JP 11-249623 A further discloses a source driver for generating control signals (A and B) for controlling the polarity of the offset voltage of an output amplifier based on an output timing control clock (CL1) for outputting display data stored in a data latch circuit to the signal line of the liquid crystal display panel and a frame period recognition signal (FLMN) for recognizing each frame period, thereby inverting the polarity of the offset voltage every two horizontal lines during each frame period and every two frame periods (see, for example, paragraphs [0017] and [0055] and FIG. 24). The output timing control clock (CL1) and the frame period recognition signal (FLMN) are used to generate the control signals (A and B), so a spatial cycle at which the polarity of the offset voltage is inverted is fixed to the two horizontal lines in the circuit disclosed in JP 11-249623 A.
The technique for inverting the polarity of the offset voltage every predetermined number of lines as disclosed in JP 11-249623 A is certainly effective for the improvement of image quality. The inventor(s) of the present invention found that the conventional source driver disclosed in JP 11-249623 A has a problem in that, when the spatial cycle at which the polarity of the data signal is inverted is made variable, the image quality for each usable spatial cycle cannot be satisfactorily maintained. For example, there is the case where a user desires a source driver adapted to both the 1H inversion drive method and the 2H inversion drive method. According to the conventional source driver, an image cannot be displayed with satisfactory quality for both the 1H inversion drive method and the 2H inversion drive method. This is because the cycle at which the polarity of the offset voltage is inverted is fixed in the conventional source driver. In the case of the 1H inversion drive method (such as the dot inversion drive method), it is suitable to fixedly invert the polarity of the offset voltage every two lines as in the conventional source driver. However, in the case of the 2H inversion drive method, such fixed inversion is not suitable.
For example, as shown in FIG. 1, assume that a data signal is generated by an output amplifier which has two states, that is, a state “A” in which the polarity of the offset voltage is “positive” and a state “B” in which the polarity of the offset voltage is “negative” and can generate any of data signals whose polarities are positive and negative. Note that, in an actual case, when the output amplifier can have two states, a state in which the polarity of the offset voltage is “positive” is unknown.
The output amplifier can generate one of four types of data signals as described below.
Type 1: both of polarity of data signal and polarity of offset voltage are positive (upward direction arrow in state “A”)
Type 2: polarity of data signal is negative and polarity of offset voltage is positive (downward direction arrow in state “A”)
Type 3: polarity of data signal is positive and polarity of offset voltage is negative (upward direction arrow in state “B”)
Type 4: both of polarity of data signal and polarity of offset voltage are negative (downward direction arrow in state “B”)
In FIG. 1, a common voltage VCOM indicates a voltage level of a common electrode of the liquid crystal display panel. According to the studies made by the inventor(s) of the present invention, in order to improve the quality of an image, it is suitable to supply the four types of data signals to pixels of the liquid crystal display panel in a spatially uniform manner.
Because the spatial cycle at which the polarity of the offset voltage is inverted is fixed to the two horizontal lines, the source driver described in JP 11-249623 is suitable for the 1H inversion drive method but not for the 2H inversion drive method. FIGS. 2A and 2B show types of data signals supplied to respective pixels during each frame period in the case where the source driver described in JP 11-249623 performs the 1H inversion drive method (dot inversion drive method) and in the case where the source driver performs the 2H inversion drive method. In FIGS. 2A and 2B, symbols “↑A”, “↓A”, “↑B”, and “↓B” have the following meanings.
“↑A”: pixel to which data signal whose polarity is positive is supplied from output amplifier having state “A” (that is, pixel to which data signal of “type 1” is supplied)
“↓A”: pixel to which data signal whose polarity is negative is supplied from output amplifier having state “A” (that is, pixel to which data signal of “type 2” is supplied)
“↑B”: pixel to which data signal whose polarity is positive is supplied from output amplifier having state “B” (that is, pixel to which data signal of “type 1” is supplied)
“↓B”: pixel to which data signal whose polarity is negative is supplied from output amplifier having state “B” (that is, pixel to which data signal of “type 2” is supplied)
Note that, according to the operation shown in FIGS. 2A and 2B, the state of the output amplifier is switched every two lines and two frame periods.
As shown in FIG. 2A, when the 1H inversion drive method is performed, the four types of data signals appear in a pixel column. For example, during a first frame period, the types of data signals supplied to respective pixels located in the leftmost column are “↑A”, “↓A”, “↑B”, and “↓B” in sequence. However, as shown in FIG. 2B, when the 2H inversion driye method is performed, only two types of data signals appear in a pixel column. For example, during the first frame period, the types of data signals supplied to the respective pixels located in the leftmost column are “↑A”, “↑A”, “↓B”, and “↓B” in sequence, so pixels in which the types of the data signals are “↓A” and “↑B” do not appear. Therefore, when the 2H inversion drive method is performed, the four types of data signals are not supplied in a spatially uniform manner. Thus, the 2H inversion drive method causes quality deterioration.
As described above, the conventional source driver in which the spatial cycle at which the polarity of the offset voltage is inverted is fixed has the problem that, when the spatial cycle at which the polarity of the data signal is inverted is made variable, the image quality for each usable spatial cycle cannot be satisfactorily maintained. It is preferable to solve the problem using a simple circuit.